A Predictive Model for Postoperative Analgesic Requirements After Abdominal Surgery

Postoperative pain remains a significant clinical challenge following abdominal surgery. Although multimodal analgesic strategies are widely implemented, many patients continue to experience moderate-to-severe pain during the early postoperative period and require repeated rescue analgesic administration. Current approaches to evaluate postoperative analgesic requirements often rely on total opioid consumption or time to first rescue analgesia. However, these methods may not fully capture the dynamic and repeated nature of postoperative pain and analgesic demand.

This study aims to develop a predictive model to characterise postoperative analgesic requirements in adults undergoing elective abdominal surgery. The study will use repeated time-to-event pharmacokinetic-pharmacodynamic modelling to analyse the timing and pattern of rescue morphine administration during the first 48 hours after surgery.

This is a prospective observational cohort study involving adult patients undergoing elective open or laparoscopic abdominal surgery under general anaesthesia. Participants will receive standard postoperative analgesic management according to routine clinical practice. No changes to clinical care will be introduced as part of the study.

Data collected will include patient demographics, surgical characteristics, anaesthetic details, postoperative pain scores, and analgesic administration records. Rescue analgesic events will be captured from patient-controlled analgesia (PCA) device records, including the timing and dose of successful morphine administrations. Individual morphine exposure will be estimated using documented dosing histories and published population pharmacokinetic parameters.

The collected data will be analysed using repeated time-to-event pharmacokinetic-pharmacodynamic modelling. The model will evaluate factors influencing the likelihood of requiring rescue analgesia over time, including temporal patterns, circadian effects, morphine exposure, and patient or surgical characteristics. Model performance will be assessed using simulation-based evaluation methods, including predictive checks and bootstrap analysis.

The expected outcome of this study is the development of a validated predictive framework for postoperative analgesic requirements after abdominal surgery. This model may provide a foundation for future personalised analgesic strategies, allowing earlier identification of patients at higher risk of inadequate pain control and supporting safer, more effective postoperative pain management.

Inclusion criteria

• Adult patients aged 18-75 years
• American Society of Anesthesiologists (ASA) physical status I-III
• Elective open or laparoscopic abdominal surgery under general anaesthesia
• Expected postoperative ward or high-dependency unit (HDU) stay of ≥ 48 hours
• Ability to provide written informed consent in Bahasa Malaysia or English
• Ability to comprehend and self-report pain using the Numerical Rating Scale (NRS, 0-10)

Exclusion criteria

• Chronic opioid therapy, defined as regular opioid use for ≥ 4 weeks prior to surgery
• Severe hepatic impairment (Child-Pugh Class B or C) or severe renal impairment (eGFR < 30 mL/min/1.73 m²)
• Known allergy or hypersensitivity to morphine or any opioid analgesic
• Cognitive impairment (documented diagnosis of dementia or delirium), language barrier, or communication deficit precluding reliable NRS self-report
• Documented history of substance use disorder (including opioid use disorder)
• Withdrawal of informed consent at any point